Intro

Banzai (short for BanzaiETL) is a document processing framework. It operates by defining pipelines. We have a PubSub model where publishers (e.g. our api) place things in a priority queue that then gets consumed by a pipeline worker. A pipeline worker subscribes to the work queue and when it finds a task that needs to be done starts working on it. A pipeline is basically a state machine that allows you to execute multiple operations on a single file. You can use it to enrich documents with stuff from the internet, convert images, replicate a database to another datastore, etc. In our case we use the pipelines to process emails, convert pdf attachments to images, resize images, process information we get from mechanical turk, etc.

Terminology

Pipeline

You can define one or more pipelines like this:

var banzai = require('banzai');
var pipeline = banzai.pipeline('order pipeline');

The pipeline object has a chainable configuration API which you can use to later configure this object.

To start with, a pipeline processes documents in a document store.

States and Transitions

Each document that enters a pipeline has a state at each moment in time.

You can trigger a handle when a document state changes. This handle is a function that receives the document and then transforms it.

You can define the pipeline states and transitions like this:

pipeline
.on('initial', initialHandler, {
next: 'order received email sent'
})
.on('order received email sent', orderEmailSentHandler, {
priority: 2
, condition: allItemsAvailable
, next: 'items available'
})
.on('order received email sent', confirmationEmailSentHandler, {
priority: 1
, next: 'items not available'
})
.on('items not available', itemsNotAvailableHandler)
.on('items available', itemsAvailableHandler, {
next: 'order placed'
})
.on('order placed', orderPlacedHandler, {
next: 'order placed email sent'
});

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initlalHandler, orderEmailSentHandler, itemsNotAvailableHandler and all the others are functions that handle the state entry. They could look something like this;

var initialHandler = function(orderDoc, done) {
email.sendOrderReceivedEmail(orderDoc, done);
};

var confirmationEmailSentHandler = function(orderDoc, done) {
done();
};

var itemsNotAvailableHandler = function(orderDoc, done) {
email.sendItemsNotAvailable(orderDoc, done);
};

It is necessary that the handlers call the done function once they are done. If an error occurs, done should be called with an error on the first argument. If not, the first argument should be null.

You can see that the confirmationEmailSentHandler handler does not pass the document to the callback function. If that happens, the previous version of the document is used.

#### Condition functions

allItemsAvailable is a condition function, which could look something like this;

var allItemsAvailable = function(orderDoc, done) {
inventory.checkAvailabilityAndReserveItems(orderDoc.items, function(err, availability) {
if (err) { return done(err); }
done(null, availability.allAvailable);
});
};

A condition function should either return a boolean or call the done function with (err, boolean). The first argument should be null if there is no error and the second argument should be a boolean, telling if the condition was met or not.

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Document Store

Each pipeline has a document store, which is where the docs that are processed are stored.

You can define the document store on a pipeline like this:

pipline.docStore(docStore);

A doc store is an object that has to have these 2 functions:

* load(docId, callback) - callback is a function with the signature (err, doc);
* save(doc, callback) - callback is a function with the signature (err, doc);

By convention, you should expect doc.id or doc._id if the document exists and update it.

There already exists a store for CouchDB named [banzai-docstore-couchdb]. You can install it like this:

$ cd
$ npm install banzai-docstore-couchdb

... and use it like this:

var banzaiDocstoreCouchdb = require('banzai-docstore-couchdb');
var docStore = banzaiDocstoreCouchdb({
url: 'http://localhost:5984'
, db: 'docs'
});
pipeline.docStore(docStore);

State Store

Each pipeline has a state store, which is where the document states are stored.

(If you don't define a state store, the document state will be stored inside a "state" property of your document).

A state store is an object that has to have these 2 functions:

* load(stateDocId, callback) - callback is a function with the signature (err, doc);
* save(stateDoc, callback) - callback is a function with the signature (err, doc);

By convention, you should expect doc.id or doc._id if the document exists and update it.

There already exists a state store for CouchDB named [banzai-statestore-couchdb]. You can install it like this:

$ cd
$ npm install banzai-statestore-couchdb

... and use it like this:

var banzaiStatestoreCouchdb = require('banzai-statestore-couchdb');
var stateStore = banzaiDocstoreCouchdb({
url: 'http://localhost:5984'
, db: 'states'
});
pipeline.stateStore(stateStore);

Work Queue

A work queue is a queue where pending transitions are put so that they are picked up by interested workers.

Each pipeline has a work queue, which is an object that has to have these functions:

* push(state_name, jobData, callback)
* state_name: a string to identify the state;
* jobData: an object to be passed onto the worker
* callback: a function with the signature (err).
* pop(state_name, callback)
* state_name
* callback: a function with the signature (err, jobData).

[banzai-docstore-couchdb]: https://github.com/pgte/banzai-docstore-couchdb
[banzai-statestore-couchdb]: https://github.com/pgte/banzai-statestore-couchdb